1. Field of the Disclosure
The disclosure generally relates to compositions useful as bone cements and, more specifically, to compositions useful in spinal surgery, a kit containing packaged components of the composition, and a method of making the composition.
2. Brief Description of Related Technology
Bone cement compositions are known to be useful in bonding or affixing an implant material to natural bone and to otherwise repair damaged natural bone. These compositions are especially useful in orthopedic and dental procedures. Although bone cement compositions enjoy wide use in the medicinal arts, these compositions need to be carefully designed depending on the surgical site at which they will be used. For example, compositions suitable for use in repairing a damaged bone in a limb may not be ideally suited for use in repairing damaged teeth. Similarly, compositions useful in repairing a limb or a tooth may not be ideally suited for surgically repairing the spinal column.
Spinal surgery is complex and risky given the proximity of the surgical site to the spinal cord and major organs. Conventional spinal surgery can repair fractured vertebrae and restore a patient's height, but it is highly invasive and involves significant risks. These surgeries involve making long incisions in the patient's chest or back so that metal instruments can move the fractured bones back into their normal positions. However, the instruments used in these surgeries are not optimally designed to manipulate deteriorated bone. The bones are then held in place by metal implants, which in the case of elderly patients with osteoporosis, can fail to hold due to the softness of the inner cancellous bone to which the implants are affixed or bonded. Consequently, conventional spinal surgery is performed in rare circumstances. Even when it is performed, the invasiveness of the procedure can result in damage to the spinal cord, pain, infection, and other disorders requiring post-operative, corrective procedures.
Recent developments in spinal surgery have made it possible to reduce the invasiveness prevalent in conventional procedures, thereby reducing some of the risks historically associated with the procedures. Among those developments are developments in compositions used to bond implants to natural bone or act as bone substitutes themselves. Another recent development is the ability to use syringe-like devices to deliver these compositions to the surgical site in a non-invasive manner. Suitable devices and delivery methods are disclosed in U.S. Pat. Nos. 6,241,734 and 6,613,054, and U.S. patent application Publication Nos. 2004/0122359 and 2002/0191487, the disclosures of which are incorporated herein by reference. In combination, conventional bone cement compositions along with the improved delivery methods these publications describe can reduce the invasiveness of conventional spinal surgery making the procedure a more viable option for patients in need of treatment.
Typically, current bone cement compositions are sold in two-part preparations containing a powder (or dry) part and a liquid (or wet) part, which, when combined, polymerize to form a hardened substance mimicking many of the physical properties of natural bone. The powder part includes a filler and a polymeric material, while the liquid part includes a reactive monomer (e.g., methylmethacrylate). The filler is a material that is bioactive on its surface to promote the natural growth of bone thereon. An example of such a filler is hydroxyapatite. Hydroxyapatite has a large surface area that undesirably absorbs the reactive monomer. Thus, the current preparations contain an excess amount of reactive monomer because a portion of the reactive monomer is absorbed into the hydroxyapatite and, therefore, does not participate in the polymerization reaction. When the two parts are combined to form the preparation and applied in practice, the monomer can undergo an exothermic reaction as the composition hardens. This heat generated by the reaction is highly undesirable because it can damage nerves, bones, and surrounding tissue, for example. This is especially undesirable in spinal surgery due to the proximity of the spinal cord. Additionally, the presence of the reactive monomer can result in undesired shrinkage of the composition after polymerization—diminishing bond quality between the composition and the bone to which the composition is bonded/applied. Moreover, the reactive monomer can be toxic to a patient if present in large amounts. Thus, the less the amount of reactive monomer necessary in the composition the less likely that the composition will experience undesired shrinkage, and the reduced incidences of patient exposure to heat and toxic materials.
Accordingly, there is a need for an improved bone cement composition including a reduced amount of reactive monomer, and a filler with a suitable surface area, particle size, and rheological properties for the intended use of the composition.